A major focus of this proposal is to continue investigation of a critical phenomenon of B-cell differentiation - the mechanisms for intraclonal development of isotype diversity. The sequence of expression of different isotypes or single cells during ontogeny is not easily explained by sequential deletions of CH genes, which is the mechanism supported by studies of the content and context of immunoglobulin heavy chain genes in mouse plasmactyomas. Experiments have been designed to explore this mechanism by (1) determining whether B cells which have expressed gamma or alpha genes can generate progeny producing IgM; (2) determing the frequency of cells expressing multiple isotypes (mu, delta, gamma or mu, delta, alpha) in immature and mature mice, and (3) estimating the lifetime of heavy chain in RNA in cells producing two isotypes. The long-term goal of these investigations is to understand the molecular mechanisms of normal B cell differentiation in order to gain insight into the pathogenesis of human immunodeficiency diseases. Hence we will continue in vitro studies of differentiation of human B cells induced by polyclonal activators and regulation of this process by T cells and macrophages.